Developing powder



United States Patent Oflice 3,441,505 Patented Apr. 29, 1969 43,9 rm.(:1. G03g 9/00, 9/02, 13/08 17.5. CI. 25262.1 8 Claims ABSTRACT OF THEDISCLOSURE Development of latent electrostatic images is accomplished byuse of a thermoplastic, conductive, non-electroscopic, polarizabledeveloper powder containing an acceptor-doner complex such as anaddition product of a polynuclear hydrocarbon and a halogen, forexample.

This invention relates to the development of electrostatic chargeimages. More particularly it relates to the development of electrostaticcharge images by means of finely divided electrostatically attractablesolid substances.

The electrophotographic process described in United States Paten No.2,297,691 and known as xerography, is a process according to which anelectrostatic charge pattern is formed corresponding to an image to bereproduced. The electrostatic image is made visible by means of anelectroscopic material which is fixed, for instance, by heating to thedeveloped material surface itself or to a second material surface, aftertransfer of the electroscopic material thereto.

In the conventionally applied development methods for electrostaticcharge images an insulating powder is used to which a charge is impartedgenerally by rubbing against carrier particles or a brush, beforebringing it into contact with the electrostatic charge image. By thisrubbing contact an electron exchange takes place between the materialsthat are rubbed against each other. The materials are selected in thetriboelectric series such that the powder will be charged to a polarityopposite to that of the latent charge image. In all these developmentmethods it is to be understood that the powder is charged before beingbrought into contact with the latent image and that it remains chargedfor some time i.e., that it is sufiiciently insulating. Suchlike powderis commonly referred to as electroscopic powder.

In the electrophotographic processes known as xerography, wherein use ismade of photoconductive layers composed of selenium and the like, thecharge image is to be found, after charging and exposure of the layer,on the surface of the photoconductive layer. Subsequently, there hasbeen a change in the art to the use of recording layers wherein thecharge images are not formed on the surface of the layer but in thelayer itself. The internal charge images may be formed for instance bycaptured charge carriers or by internal polarization. It has been foundthat the internal charge images formed by captured charge carriers orinternal polarization for instance in photoconductive polymer layers, inthe known photoconductive layers containing zinc oxide and in persistentinternal polarization layers cannot be removed by bringing their surfaceinto contact with a conductive material for instance by conducting theirsurface over well conductive metal rollers. Consequently, charge imagesthat are not formed on the surface of a layer, can be developed by meansof powder having a fairly good conductivity. Amony these powders we maymention the metallic developers described in United States patentspecifications No. 2,357,809 and No. 2,297,691.

It was found that such metallic powders are only of minor use forelectrophotography since they cannot be fixed according to the fixingmethods conventionally applied in electrophotography, i.e., by meltingon the material, and moreover give rise to the formation of roughfeeling images.

The present invention relates to solid, thermoplastic, conductive,polarizable developing powders that are suitable for the development ofelectrostatic charge images and that are capable of being fixed by heat.These developing powders are not electroscopic since they are toostrongly conductive for being charged electroscopically.

The present invention further relates to the production of visibleimages from latent electrostatic charge images by means of saiddeveloping powders.

Developing powders for electrostatic charge images according to thepresent invention are prepared with an acceptor-donor complex as thebasic substance.

By acceptor-donor complex is understood a complex compound of organic ororganic-inorganic electron donors and electron acceptors, the totalelectron configuration of which possesses an intermolecular mesomerismand a corresponding bond energy.

For more data about electron-acceptor-donor complexes we refer toGiinther-Briegleb Elektronen-Donator- Akzeptor Komplexe 1961, SpringerVerlag Berlin-Gottingen, Heidelberg, W. Germ-any; Mulliken, J. Am. Chem.Soc., 74 (1952), 811; Mulliken, J. Am. Chem. Soc., 19 (1951), 514;Mulliken, J. Chem. Phys, 56 (1952), '801.

Polarizable acceptor-donor complexes are among others the so-calledorganic molecular complexes, among which may be mentioned: the colouredmolecular complexes of polynitro compounds such as picric acid, 1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene and the like with polynucleararomatic hydrocarbons and their derivatives such as naphthalene,addition compounds of the type NC CN =0 -qulnoline (5=0.01 ohm cm.)

For details of the structure of molecular complexes we refer to M. J. S.Dewar, The Electronic Theory of Org-anic Chemistry, Oxford, ClarendonPress 1949), pp. 184-185.

The stable charge complexes known as intermolecular addition compoundsof an electron-donor (e.g., an aromatic hydrocarbon) and anelectron-acceptor (e.g., a halogen) are favoured (see among others J.Chem. Ed., October 1963, vol. 40, pp. 550-551).

Developing powders according to the present invention comprisespreferably at least 50% by weight of acceptordonor complex and possess aspecific resistivity lower than 10" ohm/cm.

Fine aluminium powder and other conductive material in powder form maybe added to the composition of the developing powder in such an amountthat the volume resistivity of the powder particles comprising theacceptor-donor complex is brought below 10 ohm/cm. and yet the powderstill can be fixed by melting.

The powder particles may also be coated with a thin layer of metal forinstance with silver metal as illustrated in Example 3 hereinafter.

For the manufacture of a developing composition according to the presentinvention the acceptor-donor complex is preferably dissolved orhomogeneously dispersed in a meltable high molecular weight compound.The presence in the developing powder of preferably not more than 50% byweight of a meltable high molecular weight compound offers thepossibility of allowing the powder to be fixed by melting exactly as isthe case with an electroscopic resin powder.

Meltable polymers which can be mixed with the acceptor-donor complexshould have a softening point preferably above 40 C. and a melting pointpreferably lower than 200 C.

Examples of suitable high molecular weight compounds are: natural resinssuch as dammar resin, elemi resins, gum mastic, gum manila, sandaracresin, microcrystalline waxes, modified natural substances such as ethylcellulose stearate, pentaerytrite polyesters, colophony resins, polymersand copolymers of ethylene, styrene and vinyl acetate, polyvinyl acetalsof formaldehyde, acetaldehyde and butyraldehyde, polyacrylates,polymethacrylates and coumarone indene resins, polycondensates such asglycerol phthalate resins or other glycerol polyesters, alkyd resins,polyethylene glycol esters, formaldehyde resins, silicone resins andpolyamides.

Ferromagnetic substances may also be mixed with said resins to make thepowder magnetically attractable so that it can be applied to thematerial to be developed by means of a magnet.

The diameter of the powder particles is of the same order of magnitudeas that of the known xerographic developing powders. Good results areobtained with particles the diameter of which is comprised between 0.020and 0.15 mm.

The developing process wherein use is made of a developing powderaccording to the present invention is quite distinguishable from theconventional electrophotographic dry developing methods in that noadditional material has to be mixed with the developing powder, topermit triboelectrical charging.

A further advantage of this developing process is that the developerdoes not become exhausted by the consumption of toner (as compared witha toner which is mixed with iron particles or glass beads as carrierparticles).

In contrast to chargeability, the polarizability of an electroscopicpowder is not a surface property but a mass property so that thedevelopment power of the powder according to the invention isindependent of atmospheric circumstances such as humidity andtemperature and in consequence thereof good results are obtained even inthe case of high relative humidity.

It has been found that by means of powder developers according to theinvention large charge areas too, are developed uniformly and with goodblackening whereas with electroscopic powders said areas are almostalways developed at the edges. The developing only powder according tothe invention need not be dusted in a dusting chamber onto the layer tobe developed as described in United States Patent 2,357,809 or appliedby sifting through a screen as described in United States Patent2,297,691. On the contrary it is desirable to bring the developingpowder as a compact mass into contact with or within the zone whereinthe electrostatic attraction of the powder to the electrostatic chargepattern is possible. This may be achieved by conducting the recordingmaterial comprising the electrostatic latent image through a containerfilled with such powder.

According to another developing embodiment the powder can also be pouredout onto the layer carrying the latent image whereupon the excess ofpowder is shaken off and then sucked off.

A preferred method for applying the developing powder according to thepresent invention is pouring the powder as a relatively thick layer ontoan endless belt or drum against which the material to be developed ispassed in contact but without counter-pressure. Since the electricityinduced by rubbing with the endless belt is immaterial in the mechanismof this process, and the triboelectric properties are of no importanceit is possible to choose for said belt a rough material. This materialmust be able to absorb and carry along a layer of developing powderwhich is not too thin. The use of a compact amount of developing powderaccording to the present invention is especially useful when widecharged zones are to be developed. By compact amoun is understood thatthe powder particles are closely united.

For an explanation of the developing process, it may be assumed that theelectrostatic forces, that are present in the photoconductive layer, actupon the powder particles and polarize them. By charge separation in theinterior of the powder particles, adhesion of these particles toelectrostatically charged surfaces is realized.

From this explanation another advantage of the present invention, viz.development to sharp contrast, can be explained, i.e., by the fact thatthe electrostatic field itself achieves the charging of the powder andthat uncontrollable supplemental charging caused by rubbing is avoided.

The following examples illustrate the preparation of developing powderaccording to the invention.

EXAMPLE 1 A mixture (a) of 10 parts of anthracene and 1 part of iodineis melted, stirred for 5 minutes and then cooled. The mass obtained ispulverized.

A mixture (b) of: Parts Pitch 2 Pentalyn A (trade name for an ester ofpentaerytrite and natural resins marketed by The Hercules Powder CompanyInc., Wilmington,

Del, U.S.A.) 2 Russ 810 (trade name for soot marketed by the firmLummerzheim) 1.5

is melted. To the melted mixture (b) 1.1 parts of the pulverized mixture(a) is added. After cooling the mass is ground to powder.

is melted, stirred for 5 minutes, cooled and pulverized. The particlesize required for rendering the powder suitable for being applied asdeveloping powder is reached after grinding for 2 hours in a ball mill.

EXAMPLE 3 The following ingredients are mixed while being dry, bygrinding for 1 hour in a ball mill:

Parts Hydroquinone 10 Quinone 10 Parts Hobimal P59 (trade name for amodified maleic resin esterified with polyethylene terephthalatemarketed by Adriaan Honigs Kunsthars-Industrie Zaandam, Netherlands)Nigrosine (C.I. 50,415B) Then, mixtures A and B are mixed by meltingvery slowly for 30 minutes. After cooling the mixture is pulverized andground for 2 hours in a ball mill.

The powder obtained is brought while strongly stirring for 1 minute in a1% ammoniacal silver nitrate solution. Then the powder is filtered offand rinsed one time with distilled water. The powder thus treated isdried in an oven at 50 C. By this treatment the surface resistivity ofthe powder is decreased with a factor I claim:

1. 'Electrically-conductive, heat-fusible developing powder fordevelopment of electrostatic chanrge patterns present in an insulatingmaterial consisting essentially of an intimate pulverulent admixture ofa thermoplastic high molecular weight organic polymeric binder and anacceptor-donor complex being a molecular addition product selected fromthe class consisting of:

(l) a polynuclear aromatic hydrocarbon and a member of the groupconsisting of a halogen, tetracyano ethylene, and a organic polynitromononuclear compound,

(2) quinoline and tetracyano quinomethylene, and

(3) quinone and hydroquinone.

2. Developing powder according to claim 1, wherein at least 50% byweight of said acceptor-donor-complex is present in said developingpowder.

3. Developing powder according to claim 1, having a specific resistivityof less than 10" ohm cm.

4. Developing powder according to claim 1, including a minor amount ofpowder conductive metal.

5. Developing powder according to claim 1, wherein the saidthermoplastic high molecular weight substance has a softening pointhigher than C. and a melting point lower than 200 C.

6. Developing powder according to claim 1, wherein the saidacceptor-donor-complex is a stable intermolecular addition compound ofanthracene and iodine.

7. Developing powder according to claim 1 wherein said acceptor-donorcomplex is the stable molecular addition product of hydroquinone andquinone.

8. The developing powder of claim 1, wherein said halogen is iodine andsaid polynitro compound is a member of the group consisting of1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene and picric acid.

References Cited UNITED STATES PATENTS 2,892,794 6/1959 'Insalaco25262.1 2,919,247 12/1959 Allen 252--62.1

OTHER REFERENCES Labes et 211., Organic Semiconductors ill, I. of Chem.

Physics, vol. 33, No. 3, September 1960, pp. 868-872.

LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

US. Cl. X.R.

